The present invention relates to a nuclear radiation induced displacement and ionization damage resistant semiconductor device which converts an optical image focused thereon into a time varying electrical signal. More particularly, it relates to a P-type buried channel charge-coupled device (P-BCD) which is uniquely doped and processed at minimum effective processing temperatures thereby enhancing device tolerance to exposure to nuclear radiation which otherwise would degrade the imaging performance of the device.
Electro-optical systems designed to sense images function by converting a spatially varying pattern of incident illumination falling upon a photo-sensitive surface into a time-varying electrical signal. The earliest such systems were mechanical scanners in which each picture element (pixel) was presented sequentially to an individual detector by means of rotating mirrors. The output of the detector was then a time-serial representation of the scene. Such imagers suffer from the fact that the detector is sensitive to photons from a given part of the scene only when that pixel is addressing the detector. If the scene being imaged consists of a large number of pixels, very poor overall sensitivity results.
Another class of imagers widely used in visible imaging, described in "Photoelectronic Imaging Devices" edited by L. M. Biberman and S. Nudelman (Plenum Press 1971), employs a position-controlled electron beam moving across a photo-sensitive surface in a raster fashion to detect an image focused onto that surface. Electron beams serve as nearly perfect commutator switches, sampling each of a plurality of pixels of the image in a time sequential manner. The surfaces used in such systems usually operate in a storage mode. In such a storage mode the photosensitive surface is charged by the scanning beam to a specified potential, which, in the absence of light, will be retained for a long time; that is until leakage processes give rise to a current, called "dark current", which discharges the surface. As long as the surface remains at the specified potential, it will accept no additional charge from the scanning beam.
However, light incident on the surface can give rise to currents which will discharge it, so that the beam current required to recharge it on the next pass is then proportional to the integrated photon flux incident on the photosensitive surface since the last pass. Such imagers are much more sensitive than those not using the storage mode, since the photosensitive surface is equivalent to many individual detectors, and all the light is detected, not just that incident during the brief interrogation period. All modern imagers employ this type of photon flux integration to exploit the sensitivity improvement which this mode of operation offers.
Both mechanical and electron beam scanners, however, have numerous disadvantages, and as a result, technologists have developed solid-state imaging systems using arrays of photosensitive elements for which the scanning function is accomplished by the application of electrical signals to interrogate one pixel at a time. Such scanning systems use several electrical control signals, with all electrodes of one type in an array being at the same voltage during one phase of the scanning cycle, and at a different voltage during other phases. In a carryover from electron beam tubes, such imagers operate the basic photosensitive device in the storage mode to enhance sensitivity. In addition to detection, self-scanned arrays also perform other signal processing functions, such as amplification, time delay and integration (TDI), multiplexing, anti blooming, background signal subtraction, etc. The material of choice for performing electronic functions such as these is silicon. When the spectral band in which imaging is to be done is one in which silicon can also serve as the photosensitive element, there is an ideal match, and the rapid progress in recent years in solid-state imagers for the visible spectrum attests to this fact.
Charge-coupled device (CCD) imagers are a particularly simple version of self-scanned solid state imagers and are described in "Imaging Devices Using the Charge-coupled Concept", by D. F. Barbe, Proceedings of the IEEE, Vol. 63, 38-67, January 1976. In the CCD imagers, each pixel consists of a metal-insulator semiconductor (MIS) structure, which, under appropriate bias voltages, establishes a potential minimum (well) in the semiconductor. When light is absorbed in the semiconductor, generating electron-hole pairs therein, minority carriers collect in charge packets at this potential well, while majority carriers are swept into the body of the semiconductor and merge with background carriers already there. The imager consists of a large array of such pixels, and thus can transfer charge packets in discrete time increments from one pixel to the next by forming a new potential well adjacent to the one holding the charge packet, and then eliminating the old well by changing the bias voltages that established it. Since the charge packets follow the potential wells, which are controlled by external voltages presented to the CCD array in various phases, transfer can be achieved in two spatial dimensions, and a two dimensional image achieved.
A CCD array is divided into vertical columns by channel stop diffusions, while electrodes running across the array at right angles to the channel stops divide the array into horizontal lines. In one version of the CCD imager, the electrodes are grouped into two sections: an integration section, in which the photon integration process takes place, and a storage section, in which the minority carrier packets are stored while being transferred to the output in a serial fashion. A horizontal readout register allows serial readout of data from the storage section. Both the storage section and the readout register are shielded from light, to prevent additional generation of carriers during the readout process.
In use, an image is focused on the optical integration section. Each pixel therein consists of several electrodes on which different voltages are applied to control the electrical potential in the semiconductor thereunder. The primary electrode of each pixel is held at a suitable voltage during a first phase of the control signal and the charge generated in the silicon by the incident light is separated by the fields existing therein. The minority carriers are swept into the potential wells beneath these electrodes and the majority carriers swept into the bulk silicon where they merge with the background carriers. Minority carriers generated by thermal processes are also swept into these potential wells, and comprise the "dark current", minimization of which is critical to successful CCD operation. To maintain the signal charges in the region where they were first generated each pixel is bounded in the direction of scan by other electrodes maintained at voltages which will not give rise to potential wells in the semiconductor, and in the direction perpendicular to the scan by the channel stop diffusions, that is regions of the semiconductor in which minority carriers are permanently prevented from entering by impurity atoms added during CCD manufacture. The amount of photon-generated charge collected in any given pixel is directly proportional to the number of photons incident on that pixel during the time over which light is integrated (the frame time), with a constant of proportionality being referred to as the quantum efficiency, ".eta.", usually considered to be spatially uniform over the array.
At the end of a relatively long integration period, the collected charges are transferred into the storage section in a short period of time by applying suitable clock pulses between a primary electrode and a plurality of secondary electrodes associated with each pixel in both the integration and storage sections. The secondary electrodes of each pixel which has been biased at voltages which constrain minority carriers from collecting under them are now biased at voltages such that potential minima form under them. Simultaneously, the bias voltage of the primary electrode is changed to a voltage such that the signal carriers are moved to the adjacent secondary electrode. By a sequence of such steps, the potential wells are moved toward the output register and the charge packets follow. Once the charge in each pixel in the integration section has been moved into the storage section, a new integration period or frame begins in the image section. During the new frame, while a new image is being integrated in the integration section, the charge constituting the first frame is moved, one line at a time, from the storage section into a serial readout register, and transferred horizontally to an output stage. The efficiency of charge transfer during each such step is called the Charge Transfer Efficiency (CTE), and is a critical parameter for CCDs. The signal arriving at the output at the time t.sub.ij is thus proportional to the light that fell on pixel ij during a particular frame, unless the transfer process causes charges to be delayed and become confused with a signal from pixel (i+1)j.
Electronic cameras featuring CCD imagers are used in a variety of military, scientific, medical, commercial, and consumer applications. For example, cam corders use a CCD imager. However, CCD vulnerability to nuclear radiation limits their use in important areas such as space surveillance, nuclear waste handling, scientific instrumentation, etc. During their early development period through the 1970s and 1980s, CCDs were found to suffer from the same radiation damage mechanisms which afflict other semiconductor devices using metal-insulator semiconductor (MIS) configurations, that is the buildup of electrical charge in the insulating layer and of surface states at silicon-insulator interfaces as a result of exposure to ionizing radiation. Early radiation effects research and development concentrated on such problems, and substantial progress was made in improving the CCD resistance to ionizing radiation, exposure to which is measured in terms of the total dose of ionizing radiation that has been absorbed, total dose usually being measured in rads (silicon).
As described in "CCDs in Astronomy" by J. Janesick, et al, Astronomical Society of the Pacific, Volume 8, Chapter 4, September 1989, as semiconductor fabrication technology advanced, larger arrays of pixels became possible which required that the minority carriers making up the charge packets be transported greater distances from a pixel site to the output gate (.apprxeq.1 cm, in some cases) without loss or delay. This imposed a requirement that did not apply to other MIS devices, that is that defects in the semiconductor capable of capturing and delaying minority carriers (called charge traps) be virtually eliminated from the silicon. Such traps degrade the Charge Transfer Efficiency, CTE, a parameter unique to CCD devices. CTE is usually very close to 1, so that it is easier described in terms of the Charge Transfer Inefficiency, CTI, which is defined as 1-CTE.
Process technologists have succeeded in reducing bulk defect concentrations to low levels in virgin devices. However, damage produced by nuclear radiation generates new crystalline defects, which serve as traps. The radiation damage mechanism which produces traps is called displacement damage, since it involves displacing a crystalline silicon atom from its position in the crystal lattice and forcing it into a position which it would not normally occupy. Displacement damage is different from ionization damage, but both can result from exposure to certain types of nuclear radiation. Because of the sensitivity of current CCD designs to radiation-induced charge traps, they are rendered unusable for some applications by radiation fluences which would be negligible by less demanding standards.
All modern CCD imagers take advantage of the fact that when electrons are the carrier in the charge packets, they move faster than their counterpart, holes. Thus, manufacturers of CCDs start with silicon which is P-type where electrical conduction is by the majority carrier, holes, which are positive carriers. Furthermore, the material used as the insulator in the metal insulator semiconductor (MIS) structure of which CCDs consist is silicon dioxide. These structures are then referred to as metal oxide semiconductor (MOS) devices, where MOS devices are a subset of MIS devices. Critical manufacturing decisions such as the specific insulator to be used, and the conductivity type of the starting material which will be employed determine many of the effects of nuclear radiation on the CCDs.
In order to understand the present invention it is important to consider briefly the details of nuclear radiation damage in CCDs. There is a large body of data on the response of CCDs to nuclear radiation, most of it gathered by space experimenters concerned about the naturally occurring nuclear radiation environment, see for example, SPIE Proceedings, February 1991, Volume 1447, Pages 70-86 by C. J. Dale and P. Marshall and Pages 87-108, by J. Janesick. To understand these data, consider the nature of the interactions between nuclear radiation and matter. When nuclear radiation passes through solids, energy is lost, either through ionization of the solids, or through non-ionizing energy loss (NIEL), i.e. atomic displacements. Most nuclear radiation types cause both types of damage and the partitioning between ionization and NIEL determines the mechanisms which dominate in any given environment. One MeV protons will displace about 0.5 atoms/proton in silicon and will deposit of the order of 1 Krad (Si) in ionization for 1E8 protons/sq cm. For most MOS structures, even unhardened commercial devices, 1 Krad will cause little degradation of device parameters. However, for large format CCDs, 5E7 defects/sq cm will degrade CTE by an unacceptable amount. Thus, in a proton environment, displacement damage will be a much greater threat than ionization. In a soft X-ray environment, on the other hand, displacement damage in silicon is energetically impossible, and ionization damage is the only concern. Intermediate between these extremes is the case of hard gamma rays such as those emitted by fission products. These photons can cause atomic displacements in silicon, yet their primary energy loss mechanism is electronic excitation. Since both displacement damage and ionization damage usually occur simultaneously, CCDs intended for use in a radiation environment must have "balanced" hardening, that is they must be hardened against damage mechanisms arising from both. Radiation effects scientists have made significant progress in hardening CCDs against total dose damage, but little in displacement damage hardening.
Several techniques exist for hardening CCDs against total ionizing dose, for example structural optimization, hardened circuit design, radiation tolerant operational procedures, and, finally, hardening of the manufacturing process used to build the device. Most of these require implementation by the manufacturer of the CCD, but manufacturers have been reluctant to implement them, since almost all applications for radiation hardened CCDs also require displacement damage hardening for which no techniques have been developed. The market for radiation hardened CCDs is not large enough to justify even minor changes in procedures if the resultant products are still unable to satisfy the customer's requirements. Total dose hardening methods must be understood, however, because the methods developed for displacement damage hardening must be compatible with them if balanced hardening is to be achieved.
The most succinct description of the technology for total dose hardening of CCDs is contained in a paper by N. S. Saks, et al in IEEE Transactions on Nuclear Science, NS-26, Pages 5074 et seq, December 1979. These authors built and tested CCDs in which the minority carriers making up the charge packets were holes, rather than electrons as used in most CCD imagers. Also, they used a construction technique in which the potential minimum was formed slightly below the silicon-silicon dioxide interface to avoid radiation-induced damage which concentrates at the interface. Their structures, called P buried channel CCDs (P-BCDs) used SiO.sub.2 /Si.sub.3 N.sub.4 gate insulators and show that devices made in this way did indeed have superior total dose hardness.
Unfortunately, imaging CCDs have never been fabricated with this technology so its advantages in nuclear hardness have never been verified. However, it will be shown that the Saks procedures are compatible with the new fabrication procedures, recommended for displacement damage hardness, which are the substance of this invention. Thus, balanced hardness can be achieved by the use of P-BCDs, modified as discussed below.
While most interactions between energetic particles and atoms in a solid transfer energy to an electron cloud causing excitation or ionization, a small fraction displace nuclei from their equilibrium positions in the lattice producing a displaced atom which occupies an interstitial position in the lattice and a vacancy comprising the lattice position formerly occupied by the displaced atom. These displaced nuclei are called primary knock-on atoms (PKAs). PKAs can produce additional vacancy-interstitial pairs by further collisions and if they are generated with sufficient energy, damage cascades will be formed. The fraction of the PKA energy that is dissipated in the production of displacements can be determined from the theory of energy partition.
Regardless of whether an atom is displaced as part of a damage cascade or as an isolated PKA, most of the vacancy-interstitial pairs produced annihilate each other by recombining with no permanent damage resulting. The vacancies that escape direct recombination migrate through the lattice and ultimately combine with other lattice anomalies forming one of the several possible stable defects that are capable of trapping minority carriers in silicon. For example, two vacancies may combine to form a divacancy which is stable up to about 300.degree. C. or a vacancy and a phosphorous (or oxygen) atom may form an E center (or A center) which is stable up to about 150.degree. C. (or 350.degree. C.). The vacancy itself is mobile at temperatures above 100.degree. K, and silicon crystals contain enough of the anomalies capable of combining with these vacancies to permit formation of stable defects for levels of radiation of common interest.
The importance of a given defect depends on the device characteristic being measured (i.e. dark current, CTE, etc.), and the density and electrical properties of the defect. For many applications, CTE degradation resulting from trapping of signal carriers at radiation-induced defects is the most serious problem arising from displacement damage. Analysis of the effect of radiation-induced traps on CTE shows that CCDs in which the minority carriers are electrons are adversely affected by two main classes of traps: (1) the divacancy formed when two vacancies combine to form a stable configuration and (2) the E-center formed when a vacancy and a phosphorous atom combine to form a stable configuration. These two traps are important in N-CCDs since either can trap an electron and hold it long enough to delay it relative to the main body of charge in a packet. However, in P-CCDs, the important trap would be the divacancy, since it is the only trap able to trap holes.